This invention relates to the fluorescence emission spectra of fluorescing compounds and more particularly to methods for the determination of the wavelength at which pure singlet state resonance fluorescence occurs.
First, some terms used throughout the specification will be discussed and summarily defined.
Consider a system of two electrons in a molecule. Each electron has an associated spin quantum number S, which may be +1/2 or -1/2. By the Pauli exclusion principle, two electrons occupying the same orbital must have their spins opposed. The projection of the spin quantum number for more than one electron is generally denoted by M. For the above mentioned state, therefore, M=O; this state being referred to as the singlet state. However, when one of the electrons is promoted to an upper orbital, its spin may be oriented in the same or in the opposite direction to that of the electron remaining in the original orbital. Therefore, in this state, M may equal +1, 0, or -1. This state is referred to as the triplet state.
As noted above, the invention described herein relates to the determination of the wavelength of singlet state resonance fluorescence, a term which will now be defined.
When a molecule is exposed to exciting radiation, singlet or triplet states may be promoted to excited singlet or triplet states. Spontaneous transitions from these excited states to unexcited states gives rise to light emission. Thus, in a compound having singlet and triplet states, an emission spectra will be obtained which results from both singlet to singlet, singlet to triplet, and triplet to triplet conversions or transitions. However, at a certain wavelength for a particular compound, absorption and emission of radiation will involve only singlet to singlet transitions. The wavelength at which the above mentioned transition occurs is called singlet state resonance fluorescence. At this wavelength, there are only singlet emissions, and no triplet emissions resulting from singlet-triplet conversion.
There has been intense interest in developing methods of determining the wavelength at which resonance fluorescence occurs. By means of such methods, several results are achieved.
First, the most efficient lasing line for pumping lasing dyes can be detected. Triplet states in a laser dye have considerable absorption for the laser light so that absorption of photons by the triplet states causes laser loss. Additionally, only singlet to singlet transitions are responsible for stimulating emission in the dye. Thus, excitation of triplet states in the laser dye excludes electrons with such excitations from the lasing process. Accordingly, using a wavelength at which resonance fluorescence occurs in a dye laser, wherein only singlet to singlet transitions occur, and singlet-triplet, and triplet-triplet transitions do not occur, considerably increases laser efficiency.
Second, two photon coherent states may be produced from resonance fluorescence. These are light pulses whose degrees of second-order coherence violates various inequalities thought to hold in the classical theory of light. The pulses obtained are also anti-bunched, which is light that violates the single beam inequality.
Third, a knowledge of the wavelength for resonance fluorescence permits optical bistability. Changing from singlet to triplet conditions results in bistability (atomic cooperation and resonance fluorescence). At resonance, with cavity damping larger than atomic relaxation, and incident field decreased, the approach to the one atom stationary state is monotonic and the system exhibits hysteresis.
Presently several experimental techniques have been developed to determine the quantum yield of triplet formation from the first excited singlet level. When the wavelength is detected where there is no yield of triplet formation, only singlet-singlet transitions are possible. Thus, the wavelength for resonance fluorescence is detected.
One method involves the addition of various heavy atom solvents to a lasing dye solution, which are known to either increase or decrease the rate constant for triplet formation. This method is indirect in that it infers triplet enhancement, or decrease, by measuring a change in the singlet fluorescence yield. This method is described in detail in Shafer, F. P., ed. Topics in Applied Physics: Dye Lasers., N. Y. Springer-Verlag, 1973, pp. 155-156. However, this method along with the other methods, is quite insensitive, and is highly time consuming and laborious to carry out.